MINOR: appctx/cli: remove the "tlskeys" entry from the appctx union

MINOR: appctx/cli: remove the "tlskeys" entry from the appctx union

This one now migrates to the general purpose cli.p0 for the ref pointer,
cli.i0 for the dump_all flag and cli.i1 for the dump_keys_index. A few
comments were added.

The applet.h file doesn't depend on openssl anymore. It's worth noting
that the previous dependency was accidental and only used to work because
all files including this one used to have openssl included prior to
loading this file.